Using fixed point theory for non-expansive mappings results and Mawhin's coincidence topological degree arguments, we discuss the solvability of the Dirichlet problem for the semilinear equation of the vibrating string u xx − u yy + f (x, y, u) = 0 in bounded domain with corner points. Here we simplify and complete the work initiated in [1] and we give some results related to the rotation number associated to the domain. Our results extend and improve those of Lyashenko [9], [10] and Lyashenko-Smiley [11].